Gas detecting apparatus having condition monitoring means

ABSTRACT

An operational amplifier ( 34 ) has its inverting input connected to a electrochemical gas sensor ( 38 ) for amplifying the current produced thereby in response to presence of a predetermined gas. In order to determine whether a sensor ( 38 ) is indeed present and that a sensor present is serviceable, a transient is applied to the non-inverting input of the operation amplifier ( 34 ). The presence or absence of the sensor ( 38 ) alters the transfer function of the operational amplifier ( 34 ) in respect of the test signal. If a serviceable sensor ( 38 ) is present, the gain of the operational amplifier ( 34 ) is high for the transient resulting at square pulse output. However, if a serviceable sensor ( 38 ) is not present, the gain of the operational amplifier ( 34 ) is relatively low and the transient retains its original form. Consequently the presence of a serviceable sensor ( 38 ) can be determined from the output of the operational amplifier ( 34 ) in response to the transient test signal.

This is a continuation of application Ser. No. 08/959,328 filed Oct. 28,1997, now U.S. Pat. No. 6,123,818.

FIELD OF THE INVENTION

The present invention relates to self-testing gas detecting apparatus.

BACKGROUND TO THE INVENTION

Electrochemical gas sensors typically comprise two or three electrodesseparated by an electrolyte. These sensors generate currents in responseto the presence of a gas, e.g. carbon monoxide hydrogen sulphide,sulphur dioxide, ammonia, for which they are adapted. Hitherto, faultsin such sensors, for example broken signal wires or loss of electrolyte,have been detected by applying a quantity of the gas to be detected to asensor while monitoring the sensor output. If a wire is broken or theelectrolyte has leaked away, there will be no, or at least a reduced,output current.

The need to test such sensors by applying quantities of gas has a numberof disadvantages. Staff are required to visit each sensor which is timeconsuming and undesirable if a sensor is located in a clean environmentsuch as is found in semiconductor processing plants. Also, if a sensorfails, its failure will not be detected until the next test. This ofcourse is undesirable where the sensor is used to detect a toxic gas orexplosive. Furthermore, if the gas to be detected is toxic, it isundesirable that it be deliberately released during the testing processand for domestic use, in particular, this method of testing is quiteunsuitable.

SUMMARY OF THE INVENTION

It is an aim of the present invention to overcome the aforementionedproblems.

Broadly stated, the present invention provides a gas detecting apparatusincluding connection means for providing electrical connection to anelectrochemical gas sensor, test means for generating a test signal andmeans to analyse a signal derived from the test signal to determinewhether a serviceable electrochemical gas sensor is connected to theconnection means.

A class of gas detecting apparatus to which the present invention isapplicable comprise an amplifier and connection means for connecting anelectrochemical gas sensor to the amplifier such that the amplifier isoperable to amplify the output of the sensor.

According to a first aspect of the present invention, there is providedA self-testing gas detecting apparatus comprising an electricalinterconnection for making a connection to an electrochemical gassensor, a test signal generating circuit for generating a test signal,an amplifier for processing the test signal from the test signalgenerating circuit according to a transfer function, and a signallingdevice, said interconnection being arranged for connecting anelectrochemical gas sensor as a component of the amplifier so as todetermine said transfer function, wherein the signalling device isresponsive to the processed test signal output by the amplifier tosignal a fault condition if the processed test signal is not indicativeof a serviceable electrochemical gas sensor being connnected into theamplifying circuit by said interconnection.

According to a second aspect of the present invention, there is provideda gas detecting apparatus comprising an electrical interconnection forreceiving a two-terminal electrochemical gas sensor, a test signalgenerating circuit for generating a test signal, an amplifier forprocessing a test signal from the test signal generating circuit andprocessing means responsive to the processed test signal output by theamplifier to determine whether a serviceable electrochemical gas sensoris connected to the interconnection, the interconnection being arrangedfor connecting an electrochemical gas sensor to the amplifier such thatthe transfer function of the amplifier for the test signal is influencedthereby.

The interconnection preferably comprises a socket. However, theinterconnection may comprise means to which a sensor may be convenientlysoldered. The physical nature of the interconnection is not critical tothe present invention, the key feature being the electrical relationshipbetween the sensor and the amplifier. It should be noted that the testsignal is not applied directly to the sensor and the result analysed.Instead, the presence of a serviceable sensor is determined indirectlyfrom the output of the amplifier produced in response to the testsignal.

Since, the transfer function of the amplifier for the test signal isinfluenced by the presence or absence of a serviceable electrochemicalgas sensor, the output of the amplifier, in response to the test signal,will vary depending on the presence or absence of a serviceable gassensor. The transfer function may be modified in respect of theamplifier's gain for the test signal or the phase shift introduced intothe test signal by the amplifier. Preferably, however, theinterconnection is configured such that the presence of a serviceableelectrochemical cell increases the gain of the amplifier for said testsignal.

If the test means is configured such that the test signal causes theamplifier to give a large output or saturate when a serviceableelectrochemical sensor is connected to the amplifier by theinterconnection, it is relatively simple to detect the presence of asensor, for example using a digital or analogue comparator. A thresholdfor electrolyte loss can be set by testing the level of the amplifieroutput at a predetermined period after the start of the test signal.This is because the lower levels of electrolyte will result in theoutput of the amplifier being above the predetermined threshold forshorter periods.

Preferably, the amplifier is an operational amplifier, theinterconnection is configured for connecting an electrochemical cellbetween the inverting input of the operational amplifier and earth, andthe test means is configured to apply the test signal to thenon-inverting input of the operational amplifier.

Conveniently, the test signal comprises a transient. The transient maybe generated by means to produce a voltage step and differentiator fordifferentiating the voltage step to produce the test signal. The voltagestep may be produced by a potential divider coupled between theamplifier's power supply lines. Thus, the sensor will be tested eachtime the amplifier is energised. However, a transient or ac signal maybe applied at any time. Application of an ac signal at an appropriatefrequency will result in the amplifier outputting a series of pulses.

In battery operated apparatus, power consumption is often reduced byintermittently operating circuits. Test signal generation linked toenergising of the amplifier is particularly suited to such apparatus.Advantageously, therefore, apparatus according to the present inventionincludes control means and switching means for switching the supply ofpower to the amplifier and the test means, wherein the control means isoperable to cyclically energise the amplifier and the test means.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of an electrochemical gas sensor;

FIG. 2 s a block diagram of an apparatus according to the presentinvention;

FIG. 3 is a circuit diagram of the amplifier circuit and the switchingcircuit of the apparatus of FIG. 2;

FIG. 4 is a flow chart illustrating the operation of the apparatus ofFIG. 2;

FIGS. 5(a)-5(d) are waveform diagrams illustrating the operation of thecircuit of FIG. 3;

DESCRIPTION OF PREFERRED EMBODIMENT

A preferred embodiment of the present invention will now be described,by way of example, with reference to the accompanying drawings.

Referring to FIG. 1, an electrochemical sensor 1 comprises a generallycylindrical cup 2 formed from plastics resin material. First and secondcontact pins 3, 4 extend through the base of the cup 2. A layer 5 ofpotting compound is located immediately over the floor of the cup 2. Afirst electrode structure 6 overlays the potting compound. A wad 7,comprising a roll of glass fibre textile, sits on top of the firstelectrode structure 6. The wad 7 is soaked in an electrolyte. Adisc-shaped cap 8 is dimensioned to plug the open end of the cup 2. Thecap 8 has an axial, centrally located hole 9 to allow gas to be sensedto pass into the cup 2. A first wire (not shown) extends from the firstcontact pin 3 and overlays the first electrode structure 6. A secondwire (not shown) extends from the second contact pin 4, up the inside ofthe cup 2, and between the wad 7 and the second electrode structure 11to provide a connection thereto.

An aperture 14 is provided in the side wall of the cup 2. This aperture14 is stopped with a plug 15.

The first electrode 6 comprises a disc of gas-permeable PTFE, coated onone face with platinum black. The coated face forms an electrode and, inthe assembled sensor 1, contacts the wad 7. The second electrodestructure 11 has the same construction and its coated face is also incontact with the wad 7 in the assembled sensor 1.

The second electrode 11 allows the passage of gas. However, it preventsthe electrolyte escaping through the hole 9 in the cap 8. The wad 7 actsas a wick to ensure that, whatever the orientation of the sensor, theelectrode structures 6, 11 remain in contact with the electrolyte.

Referring to FIG. 2, a gas detecting apparatus comprises a sensorcircuit 21, a microcomputer 22 for analysing the output of the sensorcircuit 21, a 3V battery 23, a 28 switching circuit 24 for selectivelyapplying power to the sensor circuit 21 in dependence on a switchingsignal from the microcomputer 22, an alarm circuit 25 and a loudspeaker26 connected to the alarm circuit 25.

The microcomputer 22 comprises a microprocessor 27, a read-only memory(ROM) storing a control program, a random-access memory (RAM) 29 forstoring data and an analogue-to-digital converter (ADC) 30. Thecomponents of the microcomputer 22 are connected by a data and addressbus 31. The ADC 30 receives as its input the output of the sensorcircuit 21. The microprocessor 27 is configured to have two 1-bit wideports, the first of which is connected to the switching circuit 24 andthe second of which is connected to the alarm circuit 25.

The microprocessor 27 is of a type (e.g. Motorola MC146805E2) which hasa low-power consumption WAIT mode. CPU timer-generated interrupts areused to wake up the microprocessor 27 from its WAIT state.

Referring to FIG. 3, the switching circuit 24 comprises a pnp switchingtransistor 32 and a resistor 33 connected between the base of thetransistor 32 and the first 1-bit wide port of the microprocessor 27.The emitter of the transistor 32 is connected to the positive terminalof the battery 23.

The sensor circuit 21 includes an operational amplifier (op-amp) 34, afeedback resistor 35 connected between the output and the invertinginput of the op-amp 34, a resistor 36 connected between the invertinginput of the op-amp 34 and earth, a socket 37 for receiving a plug-inelectrochemical gas sensor 38 and connecting it in parallel withresistor 36, a resistor 39 connected between the non-inverting input ofthe op-amp 34 and earth, a potential divider comprising twoseries-connected resistors 40, 41, and a capacitor 42 connecting thecentral node of the potential divider to the non-inverting input of theop-amp 34. One end of resistor 40 is connected to the collector of theswitching transistor 32 and one end of resistor 41 is connected toearth. The positive supply terminal of the op-amp 34 is also connectedto the collector of the switching transistor 32.

The operation of the gas detecting apparatus will now be described.

Referring to FIG. 4, when the microprocessor 27 becomes active, itoutputs a 0V signal from the first 1-bit wide port (step s1) to turn onthe switching transistor 32.

This applies power to the sensor circuit 21. The resistors 40, 41 of thepotential divider have values in a ratio in the region of 99:1.Consequently, the voltage at the central node of the potential dividerrises rapidly from 0V to approximately 30 mV FIG. 5(a)) when theswitching transistor 32 is turned on. This induces a correspondingvoltage increase, the test signal, on the other side of the capacitor 42across the resistor 39 (FIG. 5(b)). The voltage across the resistor 39then decays exponentially. In other words, the capacitor 42 and theresistor 39 form an imperfect differentiator. The RC time constant ofthe resistor 40 and the capacitor 42 should be much less that that ofthe resistor 39 and the capacitor 42.

If no gas sensor 38 is plugged into the circuit, the gain of the op-amp34 is determined by the ratio of the values of the resistor 35 and theresistor 36 (the well-known formula: A_(v)=R_(f)/R_(s)). This ratio isof the order of 10. Accordingly, the 30 mV peak signal across theresistor 39 will be amplified and output to the ADC 30 with a peak valueof 300 mV (FIG. 5(c)).

At this point, the microprocessor 27 reads the ADC 30 (step s2). Thevalue read from the ADC 30 is compared with a first threshold value,e.g. 1.5V, (step s3). With no sensor 38 present, the value read from theADC 30 will be below the first threshold and the microprocessor 27 thenoutputs a fault alarm signal to the alarm circuit 25 from the second1-bit wide port (step s4). The fault alarm signal is normally at 0V.However, in the event of a fault, the microprocessor 27 outputs pairs ofpulses, the time between pairs being significantly greater that the timebetween the pulse of a pair. The alarm circuit 25 causes the loudspeaker26 to output a tone pulse in response to each of the pulses from themicroprocessor 27.

The sensor 38 has a large capacitance and consequently very lowimpedance for the frequency domain components of the transient, that isthe test signal, appearing across the resistor 39. Consequently, thegain of the op-amp 34 is very high for the test signal. Indeed, the gainis so high that the op-amp's output saturates at, typically, 2V from thestart of the test signal until it has almost completely decayed away(FIG. 5(d)).

Thus, if a serviceable sensor 38 is present, the value read from the ADCat step s2 will be determined to be greater that the first threshold atstep s3. In this case, the microprocessor 27 waits for 1 second (steps5), to allow the output of the op-amp 34 to fall from its saturationlevel, and then reads the ADC again (step s6). Once the ADC has beenread for the second time, the sensor circuit 21 no longer needs to beactive. Accordingly, the microprocessor 27 raises the output from itsfirst 1-bit wide port to 3V (step s7) to turn off the switchingtransistor 32.

The value read from the ADC at step s6, is then compared with a secondthreshold, representing a predetermined gas concentration (step s8). Ifthe value is above the threshold, the microprocessor 27 sounds the gasalarm (step s9). The microprocessor 27 does this by outputting a seriesof equi-spaced 1-second pulses from the second 1-bit wide port.Corresponding tone pulses are then output by the loudspeaker 26.

If the second threshold has not been exceeded, the microprocessor 27enters its dormant or WAIT state (step s10). The microprocessor 27remains in this state until it is woken by an interrupt requestgenerated by its CPU timer (step s11). Once, the microprocessor 27 has“woken up”, it returns to step s1.

In the foregoing, the present invention has been described in the caseswhere a sensor is either present or absent. However, the presentinvention is also able to detect when a sensor is present but faulty.One of the failure modes of electrochemical sensors is a break in one ofthe wires leading to the electrodes. If this occurs, the capacitance ofthe sensor drops dramatically and the sensor appears as a simple opencircuit. In this case, the sensor circuit 21 will behave as if no sensorwere present.

Another failure mode of electrochemical sensors is the loss ofelectrolyte. If this occurs, the capacitance of the sensor 38 will fall,thereby reducing the gain of the op-amp 34 for the test signal. Thiswill have the effect of shortening the length of the 2V pulse output bythe op-amp 34. A threshold corresponding to an acceptable amount ofelectrolyte can be set by introducing a delay between steps s1 and s2.In such an arrangement, the longer the delay, the smaller the amount ofelectrolyte that can be lost before the fault alarm is sounded.

The present invention has been described with reference to a gas alarm.However, it is equally applicable to a gas concentration monitoringand/or recording apparatus, in which the gas alarm function is optional.Apparatus according to the present invention may be connected to acentral station by point-to-point links or over a network. In suchsystems, the fault and gas alarms would be notified to the centralstation. Neither of the alarm conditions need necessarily be indicatedor sounded locally.

What is claimed is:
 1. A self-testing gas detecting apparatus comprisingan electrical interconnection for making a connection to anelectrochemical gas sensor, a test signal generating circuit forgenerating a test signal which comprises a transient, an amplifier forprocessing the test signal from the test signal generating circuitaccording to a transfer function, and a signalling device, saidinterconnection being arranged for connecting an electrochemical gassensor as a component of the amplifier so as to determine said transferfunction, wherein the signalling device is responsive to the processedtest signal output by the amplifier to signal a fault condition if theprocessed test signal is not indicative of a serviceable electrochemicalgas sensor being connnected into the amplifying circuit by saidinterconnection.
 2. An apparatus according to claim 1, wherein saidinterconnection is configured such that the presence of a serviceableelectrochemical sensor increases the gain of the amplifier for said testsignal.
 3. An apparatus according to claim 2, including a comparator fordetermining whether the amplifier output exceeds a predeterminedthreshold, wherein the test signal generating circuit is configured suchthat the test signal causes the amplifier output to exceed saidpredetermined threshold value when a serviceable electrochemical cell isconnected to the amplifier by said interconnection.
 4. An apparatusaccording to claim 3, including means to determine whether saidpredetermined threshold is exceeded for a predetermined period.
 5. Anapparatus according to claim 1, wherein the test signal generatingcircuit comprises means to produce a voltage step and a differentiatorfor differentiating the voltage step to produce the test signal.
 6. Anapparatus according to claim 5, wherein the means to produce a voltagestep comprises a potential divider coupled between the amplifier's powersupply lines.
 7. An apparatus according to claim 6, including controlmeans and switching means for switching the supply of power to theamplifier, wherein the control means is operable to operate theswitching means to cyclically energise the amplifier.
 8. An apparatusaccording to claim 1, wherein the electrochemical gas sensor comprises:a first electrode (6); a second electrode (11); material (7) soaked inelectrolyte and situated on top of the first electrode (6), with thesecond electrode (11) overlying the material (7); a container enclosingsaid first (6) and second (11) electrodes and intermediate material (7);two contacts (3, 4) arranged to extend into and protrude out from thecontainer (2); a first (3) of the contacts (3, 4) being in contact withthe first electrode (6) and a second (4) of the contacts (3, 4) being incontact with the second electrode (11); a hole (9) extending through thecontainer (2) and positioned to allow gas to be sensed to pass into thecontainer (2); and an aperture (14) provided in a wall of the container(2) and a plug (15) sealing the aperture (14).
 9. An apparatus accordingto claim 8, comprising: a sensor circuit (21) coupled to the contacts(3, 4); a microcomputer (22) coupled to the sensor circuit (21) foranalyzing output therefrom; a switching circuit (24) coupled to themicrocomputer (22) and the sensor circuit (21) for selectively applyingpower to the sensor circuit (21) depending upon a switching signal fromthe microcomputer (22); a battery (23) coupled to the switching circuit(24); and an alarm circuit (25) coupled to the microcomputer (22).
 10. Agas detecting apparatus comprising an electrical interconnection forreceiving a two-terminal electrochemical gas sensor, a test signalgenerating circuit for generating a test signal which comprises atransient, an amplifier for processing the test signal from the testsignal generating circuit and processing means responsive to theprocessed test signal output by the amplifier to determine whether aserviceable electrochemical gas sensor is connected to theinterconnection, the interconnection being arranged for connecting anelectrochemical gas sensor to the amplifier such that the transferfunction of the amplifier for the test signal is influenced thereby. 11.An apparatus according to claim 10, wherein said interconnection isconfigured such that the presence of a serviceable electrochemicalsensor increases the gain of the amplifier for said test signal.
 12. Anapparatus according to claim 11, including a comparator for determiningwhether the amplifier output exceeds a predetermined threshold, whereinthe test signal generating circuit is configured such that the testsignal causes the amplifier output to exceed said predeterminedthreshold value when a serviceable electrochemical cell is connected tothe amplifier by said interconnection.
 13. An apparatus according toclaim 12, including means to determine whether said predeterminedthreshold is exceeded for a predetermined period.
 14. An apparatusaccording to claim 10, wherein the test signal generating circuitcomprises means to produce a voltage step and a differentiator fordifferentiating the voltage step to produce the test signal.
 15. Anapparatus according to claim 14, wherein the means to produce a voltagestep comprises a potential divider coupled between the amplifier's powersupply lines.
 16. An apparatus according to claim 15, including controlmeans and switching means for switching the supply of power to theamplifier, wherein the control means is operable to operate theswitching means to cyclically energise the amplifier.
 17. An apparatusaccording to claim 10, wherein the electrochemical gas sensor comprises:a first electrode (6); a second electrode (11); material (7) soaked inelectrolyte and situated on top of the first electrode (6), with thesecond electrode (11) overlying said material (7); a container enclosingthe first (6) and second (11) electrodes and intermediate material (7);two contacts (3, 4) arranged to extend into and protrude out from thecontainer (2); a first (3) of the contacts (3, 4) being in contact withthe first electrode (6) and a second (4) of the contacts (3, 4) being incontact with the second electrode (11); a hole (9) extending through thecontainer (2) and arranged to allow gas to be sensed to pass into thecontainer (2); and an aperture (14) being provided in a wall of thecontainer (2) and a plug (15) sealing the aperture (14).
 18. Anapparatus according to claim 17, comprising: a sensor circuit (21)coupled to the contacts (3, 4); a microcomputer (22) coupled to thesensor circuit (21) for analyzing output therefrom; a switching circuit(24) coupled to the microcomputer (22) and the sensor circuit (21) forselectively applying power to the sensor circuit (21) depending upon aswitching signal from the microcomputer (22); a battery (23) coupled tothe switching circuit (24); and an alarm circuit (25) coupled to themicrocomputer (22).